1. Field of the Invention
The present invention relates to a traveling-wave tube amplifier (TWTA) and, more particularly, to a traveling-wave tube amplifier in which the collector potential of a traveling-wave tube is set to be lower than a body potential of the traveling-wave tube potential so as to operate the traveling-wave tube amplifier.
2. Description of the Related Art
In a traveling-wave tube amplifier using a traveling-wave tube having a depressed collector, a voltage applied across the cathode and interaction circuit of the traveling-wave tube, i.e., a body voltage Vb, is generally set to be equal to a small-signal synchronous voltage Vbs, i.e., a body voltage at which a small-signal gain is maximized at an operating frequency when a cathode current is kept constant. Depending on conditions, the body voltage Vb is set to be slightly higher than the small-signal synchronous voltage Vbs or equal to a voltage Vbe at which an electronic efficiency .eta.e of the traveling-wave tube is maximized, or is set to be an intermediate voltage between the small-signal synchronous voltage Vbs and the voltage Vbe, or more.
In this case, the electronic efficiency .eta.e is a conversion efficiency from a kinetic energy of an electron beam to a radio frequency wave energy and defined by the following equation: EQU .eta.e=Po/(Vb.times.Ik)
where Po is a saturation RF output power, Vb is a body voltage, and Ik is a cathode current. Note that a small signal means that a RF output power is negligibly small with respect to an electron beam power (Vb.times.Ik).
When the body voltage Vb is set to be equal to the small-signal synchronous voltage Vbs, a high gain can be obtained. When the body voltage Vb is set to be equal to the voltage Vbe at which the electronic efficiency .eta.e is maximized or to be slightly higher than the voltage Vbe, the cathode current can be minimized, and the long operating life can be obtained.
The body voltage Vb of a conventional traveling-wave tube is defined from the above point of view. It is generally understood that the efficiency of the traveling-wave tube amplifier is determined by, except for the efficiency of a power supply and the transmission loss between the output portion of the TWT and the output portion of the TWTA, a tube efficiency .eta.t of the traveling-wave tube, i.e., a ratio of the RF output power of the traveling-wave tube to the total power consumption thereof. It is desired that the tube efficiency .eta.t of the traveling-wave tube is increased by any method so as to increase the efficiency of the traveling-wave amplifier.